Refractory ceramic fibers, such as those based on alumino-silicate chemistry, have been sold extensively for thermal and electrical insulation applications since their development in the 1950s. Rodent inhalation studies conducted in the 1980s demonstrated a level of carcinogenic potential associated with biopersistent refractory ceramic fibers. These studies have motivated the industry to develop physiological lung fluid-soluble and non-biopersistent inorganic fibers as an alternative to refractory ceramic fibers.
While candidate fibers have been proposed, the use temperature limit of these fibers have not been high enough to accommodate many of the applications to which high temperature resistant refractory ceramic fibers are used. For example, such low biopersistent fibers often exhibit high linear shrinkage at the continuous use temperatures and/or reduced mechanical properties when exposed to continuous use temperatures of 1260° C. and greater as compared to the performance of typical refractory ceramic fibers.
The high temperature resistant, low biopersistence fibers should exhibit minimal linear shrinkage at expected exposure temperatures, and after prolonged or continuous exposure to the expected use temperatures, in order to provide effective thermal protection to the article being insulated.
In addition to temperature resistance as expressed by shrinkage characteristics that are important in fibers that are used in insulation, it is also required that the low biopersistence fibers have mechanical property characteristics during and following exposure to the expected use or service temperature, that will permit the fiber to maintain its structural integrity and insulating characteristics in use.
One characteristic of the mechanical integrity of a fiber is its after service friability. The more friable a fiber, that is, the more easily it is crushed or crumbled to a powder, the less mechanical integrity it possesses. In general, inorganic fibers that exhibit both high temperature resistance and low biopersistence in physiological fluids also exhibit a high degree of after service friability. This results in a brittle fiber lacking the strength or mechanical integrity after exposure to the service temperature to be able to provide the necessary structure to accomplish its insulating purpose. Other measures of mechanical integrity of fibers include compression strength and compression recovery.
It is desirable to produce an improved inorganic fiber composition having an improved viscosity so as to be readily manufacturable from a fiberizable melt of desired ingredients, which exhibits low biopersistence in physiological fluids, low shrinkage during and after exposure to service temperatures of 1260° C. and greater and, which exhibits low brittleness after exposure to the expected use temperatures, and which maintains mechanical integrity after exposure to use temperatures of 1260° C. and greater.